1. Field of the Invention
The present invention relates to a method for preparing electrically-conductive zinc oxide and more specifically to a method which permits, at low cost, the preparation of non-toxic electrically-conductive zinc oxide which has a high whiteness, which is free of coarse particles and correspondingly has a sharp particle size distribution, which has a powder specific resistance of little dispersion because a dopant is uniformly distributed, and which has a low volume specific resistance.
2. Description of the Prior Art
Zinc oxides are useful as pigments kneadable with paints and varnishes, resins, rubbers and fibers and there has been desired for the development of zinc oxides having excellent electrical conductivity, in particular, in antistatic applications. Incidentally, electrical insulating properties of plastics become causes of various technical problems in several applications thereof. For instance, the electrical insulating properties of plastics become obstacles in, for instance, shielding electric parts from a relatively high electromagnetic field observed when a computer housing is, for instance, used or in discharging of electrically-charged parts. The electrical insulating properties of plastics also become causes of various problems in storing high performance explosives or IC parts, in preparing carpets which are subjected to an antistatic treatment or rubber products for medical use, or in preparing electrically-conductive adhesives for metals.
It has been known that a polymer can be converted into a electrically-conductive material through incorporation of electrically-conductive particles. As fine substances capable of imparting electrical conductivity to, for instance, plastics, paints and varnishes through incorporation thereof into these substances, there have been known, for instance, metal particles or carbon black particles; particles of semiconductor oxides such as zinc oxide and iodides (e.g., copper iodide); tin oxide powder doped with, for instance, antimony; zinc oxide powder doped with, for instance, aluminum; or powder of, for instance, titanium oxide and aluminum oxide coated with tin oxide; and fibrous materials such as glass fibers, alkali metal titanate fibers and titanium oxide fibers coated with tin oxide.
If metal particles or carbon black particles are used as electrically-conductive particles, polymers are blackened by the addition of such additives. This is often undesirable in most applications thereof. A polymer comprising copper iodide has a very low chemical stability and this substantially limits the application of the polymer. Moreover, antimony-doped tin oxide powder is excellent in a electrical conductivity-imparting ability, but the resulting polymer has a blue-black color tone due to the doped antimony and suffers from a problem of low whiteness. Further a problem of toxicity arises when antimony is used as a dopant. Thus, the polymers comprising antimony-doped tin oxide powder are substantially limited in the applications thereof.
Conventional methods for preparing electrically-conductive zinc oxide comprise the steps of adding specific metal compounds such as aluminum oxide to powdery zinc oxide and calcining the resulting mixture in the presence of solid carbon (see, for instance, Japanese Un-examined Patent Publication (hereinafter referred to as "J. P. KOKAI") Nos. Sho 54-161598, Sho 55-10478 and Sho 58-15068). In these methods, zinc oxide is doped with aluminum by calcining, at a high temperature, zinc oxide powder as a starting material in the presence of aluminum in a reducing atmosphere. However, the resulting zinc oxide has a tinge of grayish color due to the calcination in the reducing atmosphere and is, accordingly, insufficient in whiteness. Moreover, the calcination at a high temperature results in sintering and growth of zinc oxide particles formed and hence leads to the formation of coarse particles. In addition, these methods comprise complicated processes since powdery zinc oxide is used as a starting material and these methods require the use of a reductive calcination process and the production cost thereof is higher than that of carbon black.
Furthermore, various publications such as J. P. KOKAI Nos. Sho 56-69266, Sho 58-161923, Hei 1-126228 and Hei 3-115122 disclose methods comprising the steps of neutralizing a solution containing a zinc salt and a salt of a specific metal such as aluminum to co-precipitate these components and then subjecting the precipitates thus formed to a reductive calcination. However, these methods also suffer from the aforementioned problems of low whiteness, need for complicated processes and high production cost.